Ultrasonic processes and circuits for performing them

Details Ultrasonic processes and circuits for performing them Ultrasonic processes and circuits for performing them

1999-11-02

2001-04-24

Lateef, Marvin M. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S458000

Reexamination Certificate

active

06221017

ABSTRACT:

The invention relates to ultrasonic processes according to the introductory clause of claims
1
or
2
and circuits for performing them.
In ultrasonic technology, ultrasonic waves are irradiated in an examination area for selective graphic representation and/or evaluation of the Doppler spectrum. Combined transceiver sound heads are usually used in the processes and equipment for material testing and for examination of biological tissues. In this way, a sound frequency (f
0
), which is the same for the sending and receiving, is determined by the crystals of the oscillator and the equipment electronics. A typical 5 MHz sound head has a frequency range of about 3-7 MHz with a maximum at f
0
=5 MHz. In the same frequency range, the reflected and/or backscattered signal is received with the pulse-echo process. Such equipment and processes are also used in the examination of biological tissue with the use of ultrasonic contrast media. Signal portions lying outside the specified frequency range, such as, for example, oscillations being in a harmonic ratio to the sending frequency, are not used for the graphic representation of the examination object and other analyses, such as, e.g., Doppler measurements. In the previously known processes and equipment systems, several sound heads, which are changed during the examination, are further used to cover a sizable frequency range.
Improving the image quality by using harmonic multiples of the excitation frequency in ultrasonic microscopy is known from the bibliographical reference L. Germain, J. O. N. Cheeke, (J. Accoust. Soc. Am. E3 (1988) 942). For this purpose, however, ultrasonic waves with very high amplitude have to be irradiated to produce nonlinear oscillations in the path in the examination area, and by this nonlinearity, a transmission of energy from the oscillations with the basic frequency takes place in higher harmonic oscillations.
But such a process cannot be used in the ultrasonic examination with low frequencies, for example, in the range of 1-10 mHz of objects which are not resistant to high sound intensities, such as, especially, biological tissues.
The object of the invention is to broaden the range of use of ultrasonic processes for objects, limitedly resistant to sound intensity, especially biological tissues, for selective graphic representation and evaluation of the Doppler spectrum and to provide circuits for performing these processes.
According to the invention, this object is achieved in that a material is introduced in the examination area to be acoustically irradiated, with which nonlinear oscillations in this area are produced by irradiated ultrasonic waves, a broadband, acoustically highly damped, electrically matched ultrasonic converter with one or more controllable converter elements assembled individually or in groups, which corresponds to a frequency band which, in addition to the excitation frequency, comprises at least &agr;/2 and/or &agr;/3 and/or &agr;/4 times excitation frequency (f
0
), with &agr;=whole number, is excited for acoustic irradiations of the examination area and the excitation frequency and/or at least one of &agr;/2, &agr;/3, &agr;4 times it are evaluated from the ultrasonic signal received by the ultrasonic converter, reflected from the examination area or backscattered from the latter.
If &agr; is an even-numbered multiple of the denominator, the corresponding oscillations are the harmonics. If &agr;>the denominator, these oscillations are called subharmonics in the literature. If &agr;>the denominator, ultraharmonic oscillations are involved.
By introducing materials or media in the examination area to be acoustically irradiated, which produce a nonlinearity, it is surprisingly possible, even in low sonic intensities, which are not harmful, to obtain intensive and strongly frequency-shifted stray and/or transmission signals in addition to excitation frequency fo. These stray and/or transmission signals are intensive particularly in harmonics (2 f
0
, 3 f
0
. . . ), subharmonics (½ f
0
, ⅓ f
0
, ¾ f
0
) and ultraharmonics (3/2 f
0
, 5/4 f
0
. . . ) of the excitation frequency. With this process, irradiation can be performed with low frequencies, so that a greater penetration depth is obtained and receiving signals of higher frequencies can be evaluated.
In an advantageous way, a selective evaluation of the signal portions influenced by the fed materials or media as well as a selective display of the areas filled with these media is possible, without a previously necessary subtraction being made between two or more states recorded before and after the application of the materials or media. Especially, the Doppler effect caused can be evaluated independently of artifacts.
Nonlinear stray elements are advantageously introduced in the examination area. But in the examination area, a nonlinear ultrasonic contrast medium can also be introduced in the form of a solution or suspension and especially microbubbles or agents producing microbubbles.
The introduction of a microbubble suspension with a concentration of 10
−3
% by weight to 30% by weight of dry substance in a suspension medium leads to good results, and, surprisingly, the low lower limit of 10
−3
% by weight is attained with the process according to the invention and the circuit according to the invention.
In the process according to the invention, the sonic converter is advantageously excited by a function generator, with which HF bursts with adjustable amplitude and adjustable center frequency (f
T
) are produced in the range of 0.3 MHz to 22 MHz, preferably 1 MHz to 11 MHz, and with 0.5 to 20, preferably 1-5, periods. In this case, it has been shown as especially advantageous to evaluate frequencies which are smaller than sonic converter (transmitter) center frequency f
T
.
In the evaluation, it is advantageous, by a computer-controlled gate circuit, to select at least one period and to determine the related frequency spectrum in an analog or digital manner. In this case, the time window length and the number of periods per burst are adjusted between the optimum frequency resolution and optimum high-sensitivity resolution.
With the process according to the invention, Doppler effects can advantageously be evaluated in harmonics of the excitation frequency and in the mixed products, such as, the upper sideband in 2-frequency excitations. This permits the representation of slower flows without disturbances by movements of the vessel wall.
An improved penetration depth and/or space resolution, which is very advantageous in graphic representation and in Doppler measurements, further results in the evaluation of harmonic signal portions or signals in the upper sideband.
The circuit, according to the invention, for performing the process described above exhibits a function generator whose output is connected with the oscillator of an acoustically highly damped, electrically matched, broadband converter element by an T/R (transceiver) circuit synchronized by the function generator, which is downstream from a signal processing system.
In another embodiment of the circuit, the function generator is connected to the input of a converter whose output is connected to a signal processing system.
In the first case mentioned, the burst produced by the function generator in the “sending” circuit of the T/R switch is transmitted to the oscillator of the converter and the signal received by the converter is transmitted in the switched-on “receiving” position of the T/R circuit to the evaluating system. In the second case, the input and output are separated in the converter so that an T/R switch is not necessary.
A converter element, whose center frequency f
T
is greater than the upper limit of the operating range, is used with special advantage. This converter element is designed so that, as a function of the frequency in the frequency range below excitation or center frequency f
T
, the sonic intensity radiated by the converter element exhibits a positive first derivative with respect t

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